Among the types of touch panels are pressure-sensitive, capacitance-detecting, and magnetic-detecting touch panels. Touch panels of the pressure-sensitive type are widely used for mounting on display devices of portable information terminals, etc., because they have a simple circuit structure, do not require a special input pen, have a low power consumption, etc.
Further, among pressure-sensitive touch panels, the analog method, which uses a resistance film, is generally used, because a higher resolution can be obtained than with the digital method, which uses a membrane switch.
A touch panel is mounted on a liquid crystal display device which is the display device of a portable information terminal, thus enabling simultaneous input and display on the display screen.
For the liquid crystal display device which is the display device of a portable information terminal, a reflective-type liquid crystal display device is generally used, because it does not require a backlight, has a low power consumption, is thin and light-weight, etc.
Here, the state of light when a touch panel is mounted on a reflective-type liquid crystal display device will be explained using FIG. 23. FIG. 23 is a cross-sectional view explaining the state of light when a touch panel is mounted on a reflective-type liquid crystal device.
Since, as shown in FIG. 23, a touch panel 51 is mounted on a reflective-type liquid crystal display device 52, incoming light 53 reaches the reflective-type liquid crystal display device 52 after passing through the touch panel 51. Then, the incoming light 53 is reflected off a reflective substrate 54 of the reflective-type liquid crystal display device 52, and, after passing through the touch panel 51, reaches the user as outgoing light 55.
In other words, since the light passes through the touch panel 51 twice, the amount of light of the outgoing light 55 is the amount of light of the incoming light 53 multiplied by the square of the light transmittance of the touch panel 51. Thus the brightness of the reflective-type liquid crystal display device 52 is less than that of a device not provided with the touch panel 52.
One possible cause of this kind of reduction of the light transmittance by the touch panel is reflection at the interface between the touch panel's transparent conductive film and the layer of air between the touch panel's two insulating substrates.
This reflection at the interface between the transparent conductive film and the layer of air between the two insulating substrates is based on the loss of the amount of light accompanying reflection as the light passes through the interface between two media with different refractive indices. If the refractive index of the outer medium is n.sub.1 and that of the adjacent inner medium is n.sub.2, then the light transmittance T and reflection R of the interface will be as shown by the following equations. EQU Light transmittance T=4n.sub.1 n.sub.2 /(n.sub.1 +n.sub.2).sup.2 EQU Reflection R=((n.sub.1 -n.sub.2)/(n.sub.1 +n.sub.2)).sup.2
Note, with regard to the reflection R, that the larger the difference (n.sub.1 -n.sub.2) between n.sub.1 and n.sub.2, the larger the reflection R becomes, resulting in reduction of the light transmittance T and a darker display screen.
A method of solving this kind of problem by providing an anti-reflective film on the surface of the touch panel's transparent conductive film has been disclosed in Unexamined Japanese Patent Publication No. 8-195138/1996.
However, a drawback of the method disclosed in Unexamined Japanese Patent Publication No. 8-195138/1996 is that, since the entire surface of the transparent conductive film is provided with the anti-reflective film, the contact resistance between the two transparent conductive films is increased, and the touch panel's input sensitivity is impaired.
The touch panel's circuit design can compensate for this problem, but this entails development of a new circuit, which requires a great amount of time and incurs development expenses.